The primary use of Tor is to spoof your IP address. IP addresses are logged here for management purposes (it lets them know when someone is fucking around with alt accounts) they are within their rights to block Tor. Tor has also been known to be used in spam attacks on websites. I can fully understand why they block it.

Due to a recent incident, we've implemented a ban system to block an ip who is trying to steal from us. Chances are that someone used Tor as a proxy when they got banned and now that proxy is banned. @Brian would be the one to answer that question to the full extent though.